datalog: dl-magic-query handles mixed EDB+IDB relations (225/225)

Bug: dl-magic-query was skipping EDB facts for relations that had
rules ("rule-headed"). When a single relation has both EDB facts
and rules deriving more (mixed EDB+IDB), the rewritten run would
miss the EDB portion entirely, producing too few or zero results.

Fix: copy ALL existing facts to the internal mdb regardless of
whether the relation has rules. EDB-only relations bring their
tuples; mixed relations bring both EDB and any pre-saturated IDB
(which the rewritten rules would re-derive anyway).

1 new test: link relation seeded with 3 EDB tuples plus a
recursive rule via via/2. dl-magic-query rooted at `a` returns
2 results (a→b direct, a→c via via(a,e), link(e,c)).

lib/datalog/tests/magic.sx

@@ -197,6 +197,19 @@
         6)
 
       ;; dl-magic-query: end-to-end driver, doesn't mutate caller's db.
+      ;; Mixed EDB + IDB: a relation can be both EDB-seeded and
+      ;; rule-derived. dl-magic-query must include the EDB portion
+      ;; even though the relation has rules.
+      (dl-mt-test! "magic mixed EDB+IDB"
+        (len
+          (dl-magic-query
+            (dl-program
+              "link(a, b). link(c, d). link(e, c).
+               via(a, e).
+               link(X, Y) :- via(X, M), link(M, Y).")
+            (list (quote link) (quote a) (quote X))))
+        2)
+
       ;; dl-magic-query falls back to dl-query for built-in,
       ;; aggregate, and negation goals (the magic seed would
       ;; otherwise be non-ground).